New isolates of retroviridae have been obtained from primates and rodents. These include type C viruses isolated from baboons and a closely related genus, Theropithecus gelada; type C viruses isolated from Southeast Asian mice; a group D retrovirus isolated from a langur (Presbytis obscurus); and a distinctly new group of murine retroviridae isolated from M. cervicolor and M. caroli. With the exception of the langur virus, all of the other new isolates have been shown to be genetically transmitted in their species of origin. A type C viral phosphoprotein (p12) has been characterized and shown to specifically bind to type C viral RNA from the homologous virus. The extent but not the specificity of binding is regulated by phosphorylation and appears to be under host cellular control. Using techniques of somatic cell hybridization, a new gene on human chromosome six has been defined which regulates baboon type C viral replication in human X rodent cells. The new gene, designated Bevi, may be a preferred type C viral integration site in the human genome. BIBLIOGRAPHIC REFERENCES: Todaro, G.J., Sherr, C.J. and Benveniste, R.E.: Baboons and their close relatives are unusual among primates in their ability to release complete endogenous type C viruses. Virology 72: 278-282, 1976. Sen, A., Sherr, C.J. and Todaro, G.J.: Phosphorylation of murine type C viral p12 proteins regulates their extent of binding to the homologous viral RNA. Cell 10: 489-496, 1977.